But this is not a case of excessive lens flare. These are diffraction peaks, and if you look closely, you’ll see that all the bright objects in the JWST images have the same eight-ray pattern. The brighter the light, the more prominent the feature. Faint objects such as nebulae or galaxies tend not to see as much of this distortion. This pattern of diffraction peaks is unique to JWST. If you compare the images taken by the new telescope with the images taken by its predecessor, you will notice that Hubble has only four diffraction peaks compared to JWST’s eight. (Two of JWST’s spikes can be very faint, so it sometimes looks like there are six.) From now on you will always be able to tell the difference between a Hubble image and a JWST image: Hubble stars have four points in a cross. JWST’s stars are six in a snowflake. Thank you for your time. pic.twitter.com/BWsv2WqCqD — Hank Green (@hankgreen) July 12, 2022 The shape of the diffraction peaks is determined by the telescope hardware, so let’s start with a quick refresher of the important bits. Both Hubble and JWST are reflecting telescopes, meaning they collect light from the universe using mirrors. Reflecting telescopes have a large primary mirror that focuses light and reflects it back to a smaller secondary mirror. The secondary mirror in space telescopes helps guide this light to the scientific instruments that turn it into all the impressive images and data we see now. Both the primary and secondary mirrors contribute to the diffraction peaks but in slightly different ways. Light diffracts or bends around objects such as mirror edges. So the shape of the mirror itself can lead to these light spikes as the light interacts with the edges of the mirror. In Hubble’s case, the mirror was round, so it didn’t add sharpness. But JWST has hexagonal mirrors that result in an image with six diffraction peaks. Image: NASA There is also the secondary mirror. Secondary mirrors are smaller than the primary mirrors and are held in place some distance from the primary mirror by struts. In the case of JWST, the struts are 25 feet long. Light passing through these struts is refracted, resulting in more spikes, each perpendicular to the strut itself. In Hubble’s case, its four struts resulted in the four distinct spikes you see in the Hubble pictures. JWST has three struts that hold its secondary mirror, resulting in another six spikes. JWST with its struts during cryogenic tests on Earth. Image: NASA This is a big distortion. To minimize the number of diffraction peaks, the JWST was constructed so that four of the peaks caused by the supports overlap with four of the peaks caused by the mirror. This leaves the eight soon-to-be virtual diffraction peaks of a JWST image. Some of the spikes will appear more or less visible depending on the instrument that is also processing the light. This is most noticeable in the JWST images of the Southern Ring Nebula, which were released this week. Two JWST views of the Southern Ring Nebula. Image: NASA, ESA, CSA and STScI The image on the left was taken by JWST’s NIRCam, which collects near-infrared light. The one on the right was taken by the telescope’s MIRI instrument, which picks up mid-infrared light instead. “In near-infrared light, stars have more prominent diffraction peaks because they are so bright at these wavelengths,” says an explanation published by the Space Telescope Science Institute. “In mid-infrared light, diffraction spikes also appear around the stars, but they are fainter and smaller (magnify to spot).” If you’d like to see how the diffraction peaks at JWST work, check out the handy infographic below from NASA and the Space Telescope Science Institute: This chart contains a lot of text. For a text-based description, click here. Image: NASA, ESA, CSA, Leah Hustak (STScI), Joseph DePasquale (STScI)
